Because X-ray attenuates depending on an atomic number of an element constituting a substance and density and thickness of the substance, it is used as a probe for inspecting the inside of an object. X-ray imaging is widely used in medical diagnoses and non-destructive inspections.
In a general X-ray imaging system, an object is arranged between an X-ray source, for emitting X-ray, and an X-ray image detector, for detecting the X-ray, to capture an X-ray transmission image of the object. Each X-ray emitted from the X-ray source to the X-ray image detector is attenuated (absorbed) by an amount depending on the difference in object's properties (atomic number, density, thickness) existing on an X-ray path to the X-ray image detector. As a result, an X-ray absorption contrast image of the object is detected by the X-ray image detector and imaged or visualized. Photostimulable phosphor, and a flat panel detector (FPD) using a semiconductor circuit are widely used in addition to a combination of an X-ray intensifying screen and a film.
The X-ray absorption properties decrease as the atomic number of an element constituting a substance decreases. This causes a problem that in vivo soft tissue and soft material have small X-ray absorption properties, so sufficient contrast for the X-ray absorption contrast image cannot be obtained. For example, articular cartilage and its surrounding synovial fluid, both constituting a human joint, are mainly made of water, so there is little difference between their amounts of X-ray absorption, resulting in poor image contrast.
Recently, X-ray phase imaging has been studied actively. The X-ray phase imaging obtains an image (hereafter referred to as phase contrast image) based on phase shift of the X-ray wave front caused by the difference in refraction index of an object, instead of intensity variations of the X-ray caused by the object. When the X-rays are traversing the object, the phase of the X-ray wave front is much affected compared with the amplitude of the X-ray. Accordingly, the X-ray phase contrast imaging based on the phase difference makes it possible to obtain a high contrast image even if the object has low X-ray absorption property.
A radiation imaging system for obtaining a phase contrast image is suggested for performing the above-described X-ray phase imaging (for example, see International Publication No. WO2008/102654, C. David, et al., “Differential X-ray Phase contrast imaging using a shearing interferometer”, Applied Physics Letters, Vol. 81, No. 17, October, 2002, page 3287). In the radiation imaging system, a first grating and a second grating are arranged in parallel at a predetermined interval, and a self-image of the first grating is formed at the position of the second grating. The intensity of the self-image is modulated by the second grating to obtain the phase contrast image. The phase information of the object is reflected on a fringe image obtained by the intensity modulation of the self-image.
There are various methods to obtain phase information of the object using the above-described fringe image. Fringe scanning method, Moiré interference measurement method, and Fourier transform method are known. For example, in International Publication No. WO2008/102654, the fringe scanning method is used. The fringe scanning method is a method in which image capture is performed after each translational movement while one of first and second gratings is translationally moved relative to the other by a predetermined amount smaller than a grating pitch in a direction approximately orthogonal to a grating line to obtain fringe images and then a phase differential value corresponding to an amount of X-ray phase variation is obtained based on intensity variations in each pixel data. A phase contrast image is generated based on the phase differential value. The fringe scanning is applied not only to the X-ray, but also to an imaging apparatus using laser (see Hector Canabal, et al., “Improved phase-shifting method for automatic processing of moiré deflectograms” Applied Optics, Vol. 37, No. 26, September 1998, page 6227).
In the moiré interference measurement method, moiré fringes caused by a minute difference between a self-image of a first grating and a second grating are detected to obtain an amount of the X-ray phase shift based on distortion in the shape of the moiré fringes. There is no need to translationally move the grating as in the fringe scanning method. The Fourier transform method, as with the moiré interference measurement method, eliminates the need for the translational movement of the grating. The Fourier transform method is to obtain a phase differential image by obtaining spatial frequency spectrum through Fourier transform of the above-described moiré fringes and then separating spectrum corresponding to carrier frequency from the spatial frequency spectrum to perform inverse transformation.
Because the radiation imaging system disclosed in International Publication No. WO2008/102654 uses Talbot effect, a distance between the first grating and the second grating needs to be set at a value equal to Talbot length. Thus, there is a disadvantage that a grating arrangement is restricted. To solve this problem, it is known to set a distance between the first and second gratings without reference to the Talbot length by reducing X-ray diffraction at the first grating not to generate the Talbot interference so as to form a projection image of the X-ray passing through the first grating (see Chinese Patent Publication No. 101532969).
The radiation imaging system of Chinese Patent Publication No. 101532969 discloses fringe scanning method and moiré interference measurement method as methods for obtaining phase information of an object using fringe images. For the moiré interference measurement method, to surely detect the moiré fringes with an image detector, it is necessary to make the moiré fringes determined by the grating pitches of the first and second gratings larger than the pixel size of the image detector.
International Publication No. WO2008/102654 and Chinese Patent Publication No. 101532969, however, do not disclose conditions to obtain the intensity variations from each pixel in the fringe scanning method. In the fringe scanning method, imaging is performed based on a phase difference of intensity variations in each pixel between the case where an object is present and the case where an object is absent. Therefore, it is necessary to surely obtain the intensity variations from each pixel. If the intensity variations are not obtained sufficiently, accuracy of the phase differential image is degraded. As a result, a good phase contrast image cannot be obtained.
In view of the foregoing, an object of the present invention is to provide a radiation imaging system capable of surely obtaining intensity variations from each pixel and consistently obtaining a good phase contrast image.